Development of specialized in vivo sensors associated with monitoring conditions related to a patient's circulatory system has created the need for a device that can be readily attached to a preplaced catheter and used to advance a sensor into the patient's vascular system. The sensor delivery device must also satisfy other requirements. Prior to use of the delivery device, the sensor must be sterile and it must be maintained free from outside bacteriological contamination. The sensor may be stored in the delivery device prior to use, or withdrawn from the catheter into the delivery device during its use. A contaminant-free environment inside the portion of the delivery device to which the sensor is exposed should thus be maintained as the sensor is advanced through the catheter, to permit withdrawal of the sensor for subsequent reuse with the same patient. Bodily fluid leakage from the catheter through the delivery device must be avoided; yet, the delivery device should permit sampling of blood pressure, withdrawal of blood samples, and introduction of medicinal fluids into the circulatory system of the patient.
Furthermore, the delivery device should control the position of the sensor relative to the distal end of the catheter to ensure that the sensor is properly exposed to a patient's bloodstream but does not extend so far beyond the end of the catheter that it is no longer protected by an anticoagulant heparin solution flowing through the catheter. For example, fiber-optic blood gas sensors used to monitor PCO.sub.2 and PO.sub.2 are designed to extend into the bloodstream only a few hundredths of an inch beyond a distal end of the catheter. Thus it is critically important that the delivery device properly position the sensor with respect to the distal end of the catheter.